Cannabis pest management works when you scout every three days, quarantine new plants, hold temperature and humidity steady, and treat an outbreak with the mildest effective option first: beneficial insects, then insecticidal soap or horticultural oil, then a product your state cannabis program allows. Most home and small grow losses come from late detection, not from a shortage of sprays.
Scout before you treat
Pests build up under the leaves and at growing tips, so a glance at the top of the canopy tells you little. Use a 10x loupe or a phone macro lens and check the underside of ten leaves per plant, sampled from the bottom, middle, and top of the plant. Log what you see and the date. A written record turns a guess into a decision.
Sticky cards catch flying adults: fungus gnats, thrips, and winged aphids. A card that fills in a week means the population is growing.
Prerequisites
- 10x loupe or clip-on macro lens
- Yellow and blue sticky cards
- A separate room or tent for new arrivals
- Beneficial insects, or a product listed by your state cannabis program
- Gloves, eye protection, and a sprayer kept only for pest control
Step-by-step
- Quarantine every new clone or seedling for 14 days in a room away from the main canopy.
- Inspect ten leaves per plant, undersides included, every three days.
- Identify the pest with magnification before you buy any treatment.
- Cut off and bag the worst-infested leaves, and remove any plant that fails to recover.
- Fix the environment: hold flowering humidity near 50 percent, keep air moving through the canopy, and stop overwatering.
- Release beneficial insects for a light infestation: predatory mites against spider mites, Steinernema nematodes against fungus gnats.
- Spray insecticidal soap or horticultural oil on leaf undersides during the dark period, then repeat at the interval on the label.
- Rotate modes of action if a second spray is needed, so resistance does not build.
- Recheck the same ten leaves 48 hours after treatment and log the result.
Pest-specific notes
Spider mites
Two-spotted mites favor hot, dry air. Stippled leaves and fine webbing mean a settled population. Lower the temperature a few degrees, raise humidity, and coat leaf undersides, where eggs and adults sit.
Fungus gnats
Larvae live in wet media. Let the top two inches of substrate dry between waterings, hang yellow cards for adults, and drench with Bacillus thuringiensis israelensis or nematodes.
Thrips
Silvery scarring and black flecks on leaves point to thrips. Blue cards catch adults. Remove damaged growth and treat the undersides on a four-day cycle for three rounds.
Aphids
Aphids cluster on new shoots and leave sticky honeydew. A firm water rinse knocks down light colonies, and soap spray handles the rest.
Prevention
- Filter intake air and keep the grow room closed to outdoor plants.
- Change clothes and shoes after yard work before entering the grow.
- Sanitize tools, pots, and trellis netting between runs.
- Keep a two-week gap between harvest and the next planting, then clean the space.
Compliance check
Cannabis is not a labeled crop on most US pesticide products. Confirm any product against your state cannabis program's approved list, follow the label rate, and never spray in flower close to harvest. When a plant cannot be saved, bag it, remove it from the site, and disinfect the area before the next plant goes in.